Metal Supramolecular Frameworks with Silver and Ditopic Bis(pyrazolyl)methane Ligands: Effect of the Anions and Ligand Substitution

A study
of the self-assembly of different silver salts and the potentially
ditopic ligands bis­(pyrazol-1-yl)­(pyridine-4-yl)­methane (L1) and bis­(3,5-dimethylpyrazol-1-yl)­(pyridine-4-yl)­methane (L2) was undertaken and species’ with discrete or polymeric
structures were obtained. Reactions with L1 always yielded
derivatives that had a box-like cyclic dimer structure with two metal
centers and two ligands. However, on using ligand L2, the
resulting structure strongly depended on the coordination ability
of the anion. On using anions that have coordinating ability (X =
NO3–, CF3SO3–, PF2O2–),
a similar box-like structure was formed. However, the use of anions
with lower coordinating ability (X = PF6–, ClO4–) led to coordination polymers
with a helical disposition. Spontaneous resolution was observed in
the two compounds, and only one enantiomer was present in the crystals
studied. Solid-state circular dichroism spectroscopy was applied to
bunches of crystals, and the results indicate that growth of single
colonies of homochiral crystals starting from single nucleation points
may take place. When the helical complexes were recrystallized, box-like
cyclic dimers were obtained. The presence or absence of methyl groups
on the pyrazolyl rings had a strong influence on the supramolecular
structure. In solution, the box-like dimers containing L1 were more stable than those of L2.